An MRI system irradiates electromagnetic waves on a subject stayed in a uniform static magnetic field generated by a magnet to excite nuclear spins in the subject, then receives nuclear magnetic resonance signals which are electromagnetic waves generated by the nuclear spins, and forms an image of the subject. The irradiation of electromagnetic waves and reception of nuclear magnetic resonance signals are attained with an RF coil which transmits or receives electromagnetic waves of radio frequency (RF), and transmitting coils, receiving coils and coils serving as the both having various shapes suitable for MRI systems have been developed. For RF coils which performs irradiation of electromagnetic waves and detection of magnetic resonance signals, improvement in irradiation efficiency and irradiation homogeneity as well as improvement in receiving sensitivity and homogeneity of sensitivity distribution are desired.
When nuclear spins induced in a subject are excited, a coil showing a homogenous sensitivity distribution is needed for accurate imaging of an imaging region. Birdcage type RF coils and multiple patch resonators are known as coils having a homogenous sensitivity region (for example, Patent documents 1 and 2).
A so-called birdcage type RF coil such as one described in Patent document 1 is usually formed on an RF base of cylindrical shape, and comprises linear conductors (rungs) extending along the axial direction of the cylinder (z-axis direction), and circular conductors (rings) locating at the ends of the foregoing conductors. It is referred to by using the number of rungs, for example, “16-rung birdcage coil”. In the case of high pass RF coil, capacitors are disposed in the aforementioned rings. It is tuned by attaching electrical components such as capacitors and diodes to constitute an RF transmitting coil. An example of high pass type birdcage type RF coil is shown in FIG. 16. In this RF coil, two loop conductors 28 and 29 are oppositely disposed so that they should have a common center axis parallel to the z-axis among the coordinate axes, and they are connected with a plurality of linear conductors 30 (there are 12 conductors in FIG. 16) parallel to the z-axis among the coordinate axes. A plurality of the linear conductors 30 is disposed at equal intervals. The direction of the z-axis among the coordinate axes and the direction 100 of the static magnetic field generated by a magnet of MRI system are the same. A plurality of capacitors Cr are disposed between connecting points of the linear conductors 30 and the loop conductors 28 and 29, and an electric power supply point 35 is disposed at one of the capacitors.
Birdcage type RF coils have an advantage that tuning thereof is easy, and therefore they are widely used in horizontal magnetic field type MRI systems. However, they have a problem that, when the frequency to be used becomes higher with use of higher intensity of magnetic field in MRI systems, the Q values of coils are decreased. In particular, for use in a region where resonance frequency of hydrogen nucleus exceeds 160 MHz as in MRI systems having a static magnetic field intensity of 4 teslas or higher, size of a large coil such as RF transmitting coils exceeds the wavelength, thus decrease of the Q value becomes marked, and it becomes difficult to use such a coil. For this reason, use of birdcage type RF coil as an RF coil for transmission in MRI systems for imaging of human body is limited to those in MRI systems having a static magnetic field intensity up to about 3 teslas.
QD (Quadrature Detection) method is known as a method for improving irradiation efficiency and receiving sensitivity of an RF coil. The QD method is a method of detecting magnetic resonance signals by using two RF coils disposed so that axes thereof should be orthogonal to each other. If magnetic resonance signals are detected by the QD method, signals are detected by the RF coils with phases different by 90 degrees. Synthesis of these detected signals theoretically improves SN ratio √{square root over ( )}2 times compared with the case where the signals are received with one RF coil. Moreover, since circularly polarized waves are irradiated for irradiation of a high frequency magnetic field, electric power is halved, and thus heat generation due to high frequency heating in human body can be reduced. Furthermore, the QD method is also effective in view of homogeneity of images to be obtained, and it can improve sensitivity homogeneity for the x-y plane. It is easy to carry out the QD method with a birdcage type RF coil, because of the structural symmetry thereof. By disposing two electric power supply ports for transmitting and receiving signals at positions orthogonal to each other in the direction from the center, it becomes possible to perform transmission and reception by the QD method with one coil. An example of the birdcage type RF coil of FIG. 16 which is made to enable the QD method is shown in FIG. 17. In this coil, electric power supply ports 35-1 and 35-2 are disposed at positions orthogonal to each other in the direction from the center.
Birdcage type RF coils also have an advantage that they generally show high homogeneity of sensitivity distribution along the x-axis direction and y-axis direction.
On the other hand, homogeneity of sensitivity distribution along the z-axis direction is generally proportional to the length of the rungs. In order to obtain correct image of an imaging region, it is desired that non-homogeneity of sensitivity (non-homogeneity of RF power) in the imaging region should be 30% or less at the time of excitation. In order to make the non-homogeneity of sensitivity in an imaging region 30% or less at the time of excitation by using a birdcage type RF coil, the length of rungs is required to be about 1.5 times longer than the size of the imaging region along the z-axis direction. For example, when the imaging region has a length of 35 cm along the z-axis direction, the rung length is required to be 53 cm.
In the multiple patch resonator described in Patent document 2, a plurality of linear conductors (rungs) extending along the z-axis direction are disposed inside a cylindrical RF shield at equal intervals, and the rungs and the RF shield are connected via capacitors. The ring parts used in birdcage type RF coils do not exist in multiple patch resonators. Since multiple patch resonators can maintain the Q values of coils to be high even if the frequency to be used becomes higher in MRI systems having a higher magnetic field intensity, they have an advantage that they can be used even in MRI systems utilizing a static magnetic field intensity exceeding 3 teslas. However, tuning of multiple patch resonators is more complicated compared with that of birdcage type RF coils. This is because electromagnetic independency of the multiple rungs becomes higher, because the ring parts do not exist.
When the QD method is carried out with a multiple patch resonator having a coil of a small size such as a coil for imaging head, transmission and reception of circularly polarized waves are possible only by using two electric power supply ports disposed at positions whose directions from the center are orthogonal (orthogonal positions). However, in order to carry out transmission and reception of circularly polarized waves with a coil having a large size such as a coil for imaging whole body (transmitting coil), it is common to transmit and receive pulses from four electric power supply ports locating at positions each of which shifts by 90 degrees from the adjacent position with phases different by 90 degrees from that of pulse transmitted/received in the adjacent electric power supply port. This is because, because the ring parts do not exist also in this case, electromagnetic independency of the multiple rungs is high, and when electric power is supplied at the rung of the position of 0 degree, current of the rung locating at the position of 180 degrees (locating at the opposite position) is unlikely to be influenced. Increase in number of the electric power supply ports from two to four complicates adjustment at the time of use, and also increases the cost.    Patent document 1: U.S. Pat. No. 4,916,418    Patent document 2: U.S. Pat. No. 5,557,247